Glycated Hemoglobin Test

Last updated on June 13th, 2016

The glycated hemoglobin test is also referred to as the “hemoglobin A1c test”.  It is a measure of the amount of “sugar coating” is on the red blood cells as a result of diabetes mellitus.  Because red blood cells live for about 3 months, the test is a good measure of the average blood sugar values over that previous three months.  This is a test that is sometimes used to diagnose diabetes mellitus but is more commonly used to see how the blood sugar levels have been in patients already diagnosed with diabetes.

The glycated hemoglobin test is based on the amount of sugar (glucose) attached to the hemoglobin inside of red blood cells.  Hemoglobin is the protein within the red blood cells that transfer oxygen to the tissues from the lungs.  As the red blood cells are being formed, hemoglobin is a part of the making of the cells.  If the hemoglobin is exposed to high blood glucose levels during the making of a red blood cell, the end result is an elevated glycated hemoglobin test.  A normal glycated hemoglobin level is anything less than 5.7.

The glycated hemoglobin test can be used to diagnose prediabetes and type 2 diabetes.  According to experts on diabetes, the hemoglobin A1c test (glycated hemoglobin test) can be used to diagnose type 1 or type 2 diabetes in a similar way that fasting blood sugar levels and glucose tolerance tests are used in order to see if an individual has diabetes. The test involves a simple blood draw that can be done when the person is fasting or non-fasting.

hemoglobin A1c

hemoglobin A1c

The glycated hemoglobin test may, in fact, be the best test to diagnose diabetes because it can be drawn at any time during the day regardless of the fasting status of the individual.  Rather than identifying the blood sugar at any given point in time, the glycated hemoglobin test can tell what the blood sugars have been like for the past three months.  If the individual is a diabetic, this level will be elevated. A glycated hemoglobin test of between 5.7 and 6.4 percent means that the individual has prediabetes, while any glycated hemoglobin level at or above 6.5 means the individual likely has diabetes.

Why do a glycated hemoglobin test?

It is important to have periodic glycated hemoglobin testing because diabetes, especially if mild or in the early stages of the disease, has no symptoms.  People at high risk for diabetes because of things like having a family history of diabetes, having gestational diabetes in the past, or being overweight or obese should have regular glycated hemoglobin testing done to see if they have developed type 2 diabetes.  Many doctors combine the glycated hemoglobin test with a fasting blood sugar test to accurately make the diagnosis of diabetes in both low risk and high risk patients.

glycated hemoglobin test

In past years, the glycated hemoglobin test was not used for the diagnosis of prediabetes and type 2 diabetes because the test results were not standardized.  In recent years, however, the glycated hemoglobin test has become standardized so that it is a more accurate measure of the individual’s blood glucose status, making it a good test for the detection of diabetes mellitus.

Using the Glycated Hemoglobin Test for the Diagnosis of Prediabetes and Type 2 Diabetes

The glycated hemoglobin test is now used with or without also checking a fasting blood sugar test in the diagnosis of type 1 or type 2 diabetes.  The test involves a single blood draw, with the blood measured by a machine that detects the percentage of hemoglobin that is covered in glucose.  If an individual has had diabetes for at least three months, the glycated hemoglobin test can be used to make the diagnosis fairly accurately.

The diagnosis is made as follows:

  • If the person does not have type 2 diabetes, the glycated hemoglobin level will be less than 5.7 percent.
  • If the person has prediabetes, the glycated hemoglobin level will be between 5.7 and 6.4 percent.
  • If the person has diabetes, the glycated hemoglobin level will be 6.5 percent or higher.

Having prediabetes means you are at a greater risk of developing full blown type 2 diabetes within the next few years.  If the glycated hemoglobin level indicates that you are prediabetes, the test should be repeated every year until the individual shows a glycated hemoglobin level of 6.5 percent or more.  Then the glycated hemoglobin level is usually done every 3 months to monitor the effect of diabetic treatment on the blood sugar levels.

The Glycated Hemoglobin Level and Pregnancy

Pregnant women can develop gestational diabetes, which is high blood sugar levels in the latter part of the pregnancy.  In such cases, the woman will not have had high blood sugar levels for at least three months so the glycated hemoglobin level will be inaccurate.  The only time a glycated hemoglobin is helpful in pregnancy is to see if the woman had preexisting diabetes before becoming pregnant.

For gestational diabetes, a better test is the one-hour oral glucose tolerance test or OGTT.  This test involves a single blood sugar measurement taken after the woman drinks a 50-gram load of a syrupy liquid containing glucose.  If this test is abnormal, the glycated hemoglobin level and a 3-hour glucose tolerance test can be performed to confirm the diagnosis of gestational diabetes or preexisting diabetes.

Using other Blood Glucose Testing

The glycated hemoglobin test is just one way of diagnosing type 2 diabetes.  Other tests often used to make the diagnosis include having a fasting blood sugar test and doing a 3-hour glucose tolerance test in order to see if an individual has diabetes.  The test used to diagnose diabetes depends on the preference of the doctor.  Some doctors are still using the older testing measures for type 2 diabetes, while others have turned to the glycated hemoglobin test (with or without also doing a fasting blood sugar test) to make the diagnosis of diabetes in their patients.

Comparing the Glycated Hemoglobin Test and the Fasting Blood Sugar Test

Sometimes a person can have a fasting blood sugar test that shows the presence of diabetes but the person will have a normal hemoglobin A1c test.  The reverse can also happen in which the glycated hemoglobin test is abnormal and yet the fasting blood sugar test is normal.  Because this can happen, the doctor may do both tests to check for diabetes.

It is possible that a fasting blood sugar test is elevated but, if the person hasn’t had diabetes for three months or more, the glycated hemoglobin test will still be normal.  This is why the glycated hemoglobin test is sometimes repeated in another 3 months to see if it has since become elevated.

The Accuracy of the Glycated Hemoglobin Test

Blood tests, including the glycated hemoglobin test, can vary from test to test or from day to day.  The fasting blood sugar test can vary if the person is under stress, is ill, has exercised, or really isn’t fasting at the time of the test. With regard to the glycated hemoglobin test, the test may be inaccurate because the person has not had diabetes long enough to be sure that they have diabetes.

The test can also be inaccurate if the individual has abnormal blood hemoglobin types.  This means that people with thalassemia or sickle cell anemia have a variant of normal hemoglobin so that, when the glycated hemoglobin is done, the test will be unreliable.  People with these types of abnormal hemoglobin need to have a fasting blood sugar test instead of a glycated hemoglobin test when diagnosing type 2 diabetes.

Image credit wikipedia

References:

  1. The A1c Test and Diabetes
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